Temporary SIM card while visiting France

Similar to Prepaid SIM card for data in France (Paris), but I have different requirements.

I'm travelling to France for two weeks (Monaco area) and want a mobile data connection. Unfortunately it costs £9/MB for data roaming in France with my carrier (I live in Guernsey and as we aren't in the EU, there's no caps in place for the cost of roaming, even in the U.K data costs £1.50/MB to roam), which I can get down to 50p/MB but it's still not an option for me.

I therefore need a prepaid (PAYG) SIM from a carrier:

• Good coverage
• Low cost for data
• Offers data, SMS, & call bundles*
• Will work with my iPhone 5S
• Be stable enough to stream content**
• Be able to receive calls/SMS regardless of a bundle (mentioned above)

^{*Something that beats my home contract of 10,000MB, 10,000 texts, and 10,000 minutes would be great, though I don't expect anything near this.}

^{*I'm aware of restrictions regarding certain content (e.g BBC iPlayer) and have a workaround.}

I'm unsure where to purchase one from (especially since in the U.S you pay for the SIM card alone, where every carrier in the U.K and Guernsey offers free SIM cards):

Can I walk into any store and purchase a prepaid SIM, or does it have to be a supermarché (or even only the carrier's own store)?